diesels in the US
there are several reasons that there are not more diesel cars for sale here in the US.
1) demand - overall, our fuel prices are still quite low and there is consequently not a large demand for high mileage cars. at most of the stations I see diesel is also not much cheaper than base-grade regular gasoline, which means a diesel would only have improved fuel economy to offer, which the US consumer has generally shown is not a purchase priority.
2) diesel fuel supplied here in the US (primarily for commercial vehicles) has a much higher sulfur content than fuels in Europe, making it difficult to get a car to pass the stricter emissions standards we have. apparently most of the particulate emissions concerns have been overcome, it's just the sulfur-related stuff. making low-sulfur diesel the standard would add significant cost to the bottom line for millions of truckers who at this point are probably a much stronger lobbying force than the few of us who would drive diesel cars.
3) a perceived stigma associated with diesel - stinky, black smoke, clattery engines & the (in some areas, I don't see it where I live) apparent lack of lots of stations stocking it.
4) cost of certification of a unique powertrain for the US market with very low volume. generically the cost for an automaker to certify a unique drivetrain for sale in the US is around $1,000,000. this is hard to justify if you're only going to sell a small volume like MINI would if they sold the Toyota-sourced 1.4l here.
the only diesel cars for sale in the US now are the Golf/Jetta/Beetle TDi's which have been very successful here in their limited market niche.
Mercedes Benz will re-introduce a diesel E-class with the new generation of the E this fall. the current generation does not offer a diesel in the US, but the prior one did & M-B has stated that their previous diesel customers wanted it, so they are bringing it back.
personally I would love to have more diesels available here, esp some of the extremely efficient ones like the "3 liter" VW Lupo or the very powerful ones like the BMW 3 & 4 liters. I suspect the diesel Mini would also be a lot of fun.
Ben